Catalyst for selective methanization of carbon monoxide

ABSTRACT

The invention relates to a catalytic composition and to a process for selective methanization of carbon monoxide in hydrogen- and carbon dioxide-containing streams, wherein the active component used is ruthenium and the support material is a lanthanum-cerium-zirconium oxide, and to the use thereof in fuel cell systems.

This is a divisional application of U.S. application Ser. No.12/527,433, filed Aug. 17, 2009, which is a 371 of PCT/EP08/051,862filed on Feb. 15, 2008.

DESCRIPTION

The invention relates to a catalytic composition and to a process forselective methanization of carbon monoxide in hydrogen- and carbondioxide-containing streams, especially for use in fuel cell systems.

Low temperature PEM fuel cells (PEM=polymer electrolyte membrane) can beoperated only with hydrogen or hydrogen-rich gases of a defined quality.The carbon monoxide (CO) concentration in particular is a criticalparameter. It depends on the energy carrier used and on the reformingprocess used. The removal of relatively high CO concentrations ispossible with the water gas shift reaction with further formation ofhydrogen.

CO+H₂O

CO₂+H₂ ΔH=−44 kJ/mol

Since this is an equilibrium reaction, depending on the process designand temperature, a residual concentration of CO remains in the gasstream, generally in the range from 0.25 to 1.5% by volume. In the caseof use of catalysts with a high copper content, for example, CO removaldown to 2 500 ppm can be achieved. The CO content in the hydrogen-richgas must, however, be reduced further in order to prevent poisoning ofthe anode catalyst; guide values here are between not more than 10 and50 ppm.

The level of CO present in the gas stream is typically reduced down tobelow the required limits in a fine purification stage. Selectiveoxidation is currently the most common CO removal method. Selectiveoxidation has a high level of development, but possesses not only thedisadvantage of only moderate selectivity but also the necessity ofexact metering of the air supply, which results in a high level ofmeasurement and control complexity. If the necessary ratio of oxygen toCO is not maintained exactly, this can lead to high losses of hydrogen.Moreover, the narrow temperature window of generally not more than 20°C. requires complex heat management of the reactor. There is anadditional safety problem through the addition of the oxidizing agent,oxygen, to the gas. The removal of the CO by reaction with H₂(methanization) has considerable advantages over the selective COoxidation by virtue of its undemanding implementation in terms ofprocess technology.

CO methanization (hydrogenation of carbon monoxide to methane) proceedsaccording to the reaction equation:

CO+3H₂→CH₄+H₂ ΔH=−206.2 kJ/mol

A competing reaction which proceeds is the conversion of CO₂ to methane:

CO₂+4H₂→CH₄+2H₂O ΔH=−164.9 kJ/mol

The particular challenge for the selective CO methanization is that COshould be hydrogenated preferentially and not CO₂, since this wouldconsume further hydrogen. According to the thermodynamics, COmethanization is preferred over CO₂ methanization. It is known that CO₂methanization does not set in below a CO concentration limit of 200 to300 ppm in the reformate. The CO concentration in the reformate isapprox. 15 000 ppm, i.e. higher than the upper limit stated by a factorof 50. The CO₂ content of approx. 15 to 25% by volume is one order ofmagnitude above the CO content. Accordingly, a CO-selective catalyst isindispensible at low CO concentrations, as required, for example, forPEM fuel cells.

The selective methanization of CO has been known for some time. Atfirst, CO was methanized over a nickel catalyst, although CO₂ had to bescrubbed out beforehand. In 1968, a ruthenium catalyst for selective COmethanization was claimed by Baker et al. (U.S. Pat. No. 3,615,164),which involved the use of a ruthenium or rhodium catalyst on an aluminumoxide support material. Chemical Abstracts, volume 74, 1971, 35106ulikewise describes the selective methanization of CO in a gas mixturecomprising hydrogen, carbon dioxide and carbon monoxide at temperaturesin the range between 125 and 300° C. using ruthenium catalysts. U.S.Pat. No. 3,663,162 of 1972 claims a Raney nickel catalyst for thisreaction.

In EP-A-1174486, a methanization stage is combined with a unit forselective oxidation with the aim of a lower oxygen consumption and of alower CO₂ methanization rate. The catalyst used for the methanizationcomprises Ru, Pt, Rh, Pd or Ni on an aluminum oxide support.

In EP-A-0946406, two methanization stages of different temperaturelevels are connected to one another. The advantage here is said to bethat only a small amount of CO₂, if any, methanizes in the hightemperature stage, but a large proportion of the carbon monoxide isalready degraded. In the subsequent low temperature methanization, theresidual CO is removed. A noble metal catalyst is used, especially Rh,on an aluminum support.

WO 97/43207 describes the combination of a first stage for selectiveoxidation with a subsequent methanization stage with rhodium as theactive component. This combination is said to allow both processes to beoperated under optimal conditions.

Further, more recent applications, for example EP-A-1246286, in which,as the last process stage of a gas purification, a methanization reactoris connected downstream of a unit for selective oxidation for thereasons of simpler construction and of better handling, use conventionalcatalysts, predominantly based on ruthenium or nickel. JP-A-2002/068707discusses methanization catalysts applied to a refractory inorganicoxide selected from oxides of aluminum, titanium, silicon or zirconium.

EP-A-1707261 describes a process for selective oxidation of CO with Ruon a catalyst composed of mixed metal oxides, doped with lanthanides.

US-A-2005/0096212 describes selective methanization over a catalystcomposed of Ru, Rh, Ni or combinations on 8-zeolite, mordenite andfaujasite. Although the desired CO concentrations below 100 ppm areachieved in this way, the selectivity at temperatures above 190° C., atwhich the catalyst displays its activity, falls significantly below 50%.Since the hydrogenation of CO₂ destroys 3/2 as much hydrogen per mole asthe hydrogenation of CO, the requirement for maximum selectivity is veryimportant. Moreover, a viable catalytic activity is achieved only overthe very small temperature window between 170° C. and 180° C.

The prior art processes do not permit sufficient lowering of the COcontent with preservation of the CO₂ content to be ensured. Thecatalysts developed to date either do not work selectively enough or areactive only within a very narrow temperature range. The very narrowtemperature range in particular makes industrial implementation of the“selective methanization” concept very difficult. This is because, assoon as the selectivity falls, the reactor is heated, which leads tofurther methanization of CO₂ and hence to the thermal “runaway” of theprocess unit. The exothermicity of the reaction thus results inhotspots. For this reason, a wide temperature window has to be operable.Equally problematic is the adiabatic temperature increase in monolithswhen they are used as catalysts, which is often the case in practice.

For fuel cell applications in particular, the required maximum COcontent in the hydrogen-rich gas fed in and the necessary highselectivity (methanization of CO, but not of CO₂) over a widetemperature window still represents great potential for development forsuitable deactivation-resistant catalysts.

It was thus an object of the invention to provide a catalyst forselective CO methanization, which is selective and active within a widetemperature range.

The object is achieved in accordance with the invention by using, forthe selective methanization of carbon monoxide in hydrogen- and carbondioxide-containing streams, a catalytically active composition whichcomprises ruthenium as the active component and alanthanum-cerium-zirconium oxide (LaCeZr oxide) as the support material.

A catalyst which comprises a lanthanum-cerium-zirconium oxide as thesupport material and ruthenium as the active component is capable ofensuring the methanization of CO within a wide temperature range in avirtually constant selectivity over a long period of time. Conventionalcatalysts exhibit a significant decline in selectivity with increasingtemperature and prolonged run times. Employment of the inventivecatalyst requires a significantly lower level of control complexity,since the temperature window in the methanization of the CO has to becomplied with to a less exact degree. Furthermore, a catalyst whichworks well even at high temperatures can be connected directlydownstream of the prepurification stage (low temperature conversion),which is operated at about 220 to 280° C.

The invention thus provides a catalytically active composition for theselective methanization of carbon monoxide in hydrogen- and carbondioxide-containing streams, which comprises ruthenium as the activecomponent and a lanthanum-cerium-zirconium oxide as the supportmaterial.

The invention further provides for the use of this catalytically activecomposition for the selective methanization of carbon monoxide inhydrogen- and carbon dioxide-containing streams, and for use in fuelcell applications.

The embodiments of the present invention can be inferred from theclaims, the description and the examples. It is obvious that thefeatures of the inventive subject matter which have been specified aboveand are still to be explained below are usable not just in theparticular combinations specified but also in other combinations withoutleaving the scope of the invention.

According to the invention, the support material used is alanthanum-cerium-zirconium oxide (LaCeZr oxide).

The support material advantageously has a lanthanum oxide content of 0.1to 15% by weight, preferably of 5 to 15% by weight and more preferablyof 10 to 15% by weight. The cerium oxide content is advantageously 0.1to 15% by weight, preferably 0.1 to 10% by weight and more preferably 3to 7% by weight, based in each case on the weight of the overall supportmaterial.

The zirconium oxide content of the support material is advantageously 30to 99.8% by weight. In preferred embodiments, it is at a content whichsupplements the proportions by weight of the lanthanum oxide and of thecerium oxide and any further constituents, as described above, to 100%by weight in each case.

In addition to the components mentioned, the support material used inaccordance with the invention may comprise further materials usablecustomarily in catalyst chemistry for these purposes, for examplealuminum oxide. Suitable binder materials are those which have asufficiently high BET surface area. The BET surface area of theseadditionally used binder materials should advantageously be at least 120m²/g. The content of these further materials should not exceed 30% byweight, preferably 20% by weight, based in each case on the weight ofthe overall support material.

The catalytically active composition comprises ruthenium as the activecomponent. The active component is preferably present in the catalyst asthe oxide. The actual active material is then generated in situ byactivation with hydrogen.

The loading of the support material with the active ruthenium componentsis advantageously 0.1 to 20% by weight, preferably 0.1 to 10% by weightand more preferably 0.1 to 5% by weight. Further advantageous rangesare, for example, 1 to 10% by weight, 1 to 5% by weight, and also 2 to 5and 3 to 5% by weight. The figures are based in each case on the totalweight of the catalytically active composition.

A preferred composition of the catalytically active system comprises, ona lanthanum-cerium-zirconium oxide support with a lanthanum oxidecontent of 0.1 to 15% by weight and a cerium oxide content of 0.1 to 15%by weight, based in each case on the weight of the entire supportmaterial, 0.1 to 20% by weight of Ru, based on the total weight of thecatalytically active composition.

A further preferred composition of the catalytically active systemcomprises, on a lanthanum-cerium-zirconium oxide support with alanthanum oxide content of 0.1 to 15% by weight and a cerium oxidecontent of 0.1 to 15% by weight, based in each case on the weight of theentire support material, 2 to 5% by weight of Ru, based on the totalweight of the catalytically active composition.

A further preferred composition of the catalytically active systemcomprises, on a lanthanum-cerium-zirconium oxide support with alanthanum oxide content of 0.1 to 15% by weight and a cerium oxidecontent of 0.1 to 10% by weight, based in each case on the weight of theentire support material, 3 to 5% by weight of Ru, based on the totalweight of the catalytically active composition.

A particularly preferred composition of the catalytically active systemcomprises, on a lanthanum-cerium-zirconium oxide support with alanthanum oxide content of 10 to 15% by weight and a cerium oxidecontent of 3 to 7% by weight, based in each case on the weight of theentire support material, 3 to 5% by weight of Ru, based on the totalweight of the catalytically active composition.

Further embodiments in the composition of the inventive catalyst can beinferred from the examples. It is obvious that the features of thecatalyst which have been specified above and are still to be statedbelow are usable not just in the specified combinations and value rangesbut also in other combinations and value ranges within the limits of themain claim, without leaving the scope of the invention.

In addition, the active component and/or the support material can bedoped in small amounts with further elements which are usable for thesepurposes and are known to those skilled in the art, without leaving thescope of the invention.

The inventive catalyst is prepared in a customary manner, for example bydissolving the active component and any doping elements, preferably inthe form of their salts/hydrates, and then applying them in a suitablemanner, for example by impregnation, to the lanthanum-cerium-zirconiumoxide support. Thereafter, the catalyst is dried, calcined, reduced ifappropriate and passivated if appropriate.

The active components can be applied in a customary manner to thesupport material by impregnation, for example as a washcoat to amonolith. The procedure and process conditions are described, forexample, in the Handbook of heterogeneous catalysis, Vol. 1, VCHVerlagsgesellschaft Weinheim, 1997.

An alternative mode of preparation comprises the kneading of the supportmaterials with the salts/hydrates of the active elements and any dopingelements with subsequent extrusion, drying and calcination ifappropriate, reduction if appropriate and passivation if appropriate.

The kneading of the support material with the active materials and thefurther steps can be effected in a customary manner with knownapparatus.

Shaped bodies can be produced from pulverulent raw materials bycustomary methods known to those skilled in the art, for exampletableting, aggregation or extrusion, as described in referencesincluding the Handbook of Heterogeneous Catalysis, Vol. 1, VCHVerlagsgesellschaft Weinheim, 1997.

In the shaping or the application, assistants known to those skilled inthe art, such as binders, lubricants and/or solvents, can be added.

This gives rise to a catalytically active composition which isoutstandingly suitable for the selective methanization of carbonmonoxide in hydrogen- and carbon dioxide-containing streams. Dependingon the particular reaction conditions, this achieves the desiredsignificant depletion of CO to less than 10 ppm in the gas mixture withminimal loss of hydrogen.

Advantageously, the selective methanization of the CO is thus achievedwithin a temperature range from preferably 100 to 300° C.

The selective methanization of CO in a temperature range from 180 to260° C. is particularly advantageous. This temperature enables directthermal integration with the upstream low temperature conversion. Itthus becomes possible to couple the inventive methanization stagedirectly onto the low temperature conversion stage. The high activitywith equally high CO selectivity within this temperature range ensuresthat stable and in particular thermally integrated operation of thecatalyst becomes possible at all.

The catalytically active composition is thus outstandingly suitable forCO fine purifications in hydrogen- and carbon dioxide-containingstreams, more particularly for use in the generation of hydrogen forfuel cell applications.

The invention is illustrated in detail by the examples which follow, butwithout undertaking a corresponding limitation thereby.

EXAMPLES Example 1

206.3 g of ZrO₂, 29.6 g of La₂(NO₃)₂, 0.24 g of CeO₂ and 31.3 g ofaluminum oxide hydroxide (Pural SB) were mixed in a kneader andacidified with dilute HNO₃. A sufficient amount of water to give rise toan extrudable material was added. The shaped extrudates were dried andcalcined. Thereafter, this support was admixed with a RuCl₃ solutionwhose concentration was such that the end product, calcined once again,bore 3% by weight of Ru as the active material.

Example 2

279.8 g of ZrO₂, 12.5 g of CeO₂, 31.5 g La₂(NO₃)₂ and 33.3 g of aluminumoxide hydroxide (Pural SB) were used to produce a support material asdescribed in example 1. Thereafter, this support was admixed with aRuCl₃ solution whose concentration was such that the end product,calcined once again, bore 3% by weight of Ru as the active material.

Example 3

A support composed of 70% by weight of ZrO₂, 15% by weight of CeO₂, 5%by weight of La₂O₃ and 10% by weight of Al₂O₃ was admixed with a RuCl₃solution whose concentration was such that the end product, calcinedonce again, bore 3% by weight of Ru as the active material.

Example 4 Comparative Example

37.5 g of PZ2-25 H(H-ZSM-5, MFI structure type, from Zeochem,modulus=25) were initially charged with 11.8 g of aluminum oxidehydroxide (Versa 250, from UOP) in a kneader, and corroded slightly withformic acid. The mixture was admixed with water, extruded and calcined.Thereafter, this support was impregnated with a solution of rutheniumchloride hydrate and lanthanum nitrate which was such that the endproduct comprised 3% by weight of Ru and 5% by weight of La. Theextrudates were dried and calcined.

Example 5 Comparative Example

100 g of PZ2-25 H(H-ZSM-5, MFI structure type, from Zeochem, modulus=25)were initially charged with 35.6 g of aluminum oxide hydroxide (PuralSB), and corroded slightly with formic acid. The mixture was admixedwith water, ruthenium chloride hydrate and iron chloride hydrate, andextruded. After calcination, the end product comprised 3% by weight ofRu and 1% by weight of Fe.

Example 6 Comparative Example

Testing of a commercially available methanization catalyst with 5% Ru onTiO₂.

Example 7a-d

A support composed of 70% by weight of ZrO₂, 15% by weight of CeO₂, 5%by weight of La₂O₃ and 10% by weight of Al₂O₃ was admixed with an RuCl₃solution whose concentration was such that the end product, calcinedonce again, bore 5% by weight of Ru (example 7a), 4% by weight of Ru(example 7b), 3% by weight of Ru (example 7c) or 2% by weight of Ru(example 7d) as the active material.

Example 8 Reworking of Patent US 2005/096211

150 g of TZB 213 (β-zeolite, from Sud-Chemie/Tricat, modulus=12) wereinitially charged with 50 g of aluminum oxide hydroxide (Pural SB) in akneader and corroded slightly with formic acid. The mixture was admixedwith water, extruded and calcined. Thereafter, this support wasimpregnated with a solution of ruthenium nitrosylnitrate which was suchthat the end product comprised 3% by weight of Ru. The extrudates weredried and calcined.

Example 9 Reworking of Patent JP 2002/068707

432.8 g of ZrO₂ powder were initially charged with 12 g ofmethylcellulose (Walocel, from Wolff Cellulosics) in a kneader, andcorroded slightly with nitric acid. The mixture was admixed with waterand ruthenium chloride hydrate solution, and extruded. Aftercalcination, the end product comprised 3% by weight of Ru.

Example 10 Reworking of Patent JP 2002/068707

250 g of aluminum oxide hydroxide (Pura) SB) were initially charged in akneader and corroded slightly with formic acid. The mixture was admixedwith water and ruthenium chloride hydrate solution and extruded. Aftercalcination, the end product comprised 3% by weight of Ru.

Example 11 Reworking of Patent JP 2002/068707

235.5 g of TiO₂ powder were initially charged in a kneader and corrodedslightly with formic acid. The mixture was admixed with water andruthenium chloride hydrate solution, and extruded. After calcination,the end product comprised 3% by weight of Ru.

Test Conditions:

For the experiment, an electrically heated tubular reactor with a volumeof 50 ml and a diameter of 14 mm was used.

At the bottom, 4 ml of steatite spheres with a diameter of 1.8 to 2.2 mmwere installed, onto which the catalyst mixture was subsequentlyintroduced. The catalyst mixture consisted of approx. 20 ml of catalystwhich had been mixed thoroughly with approx. 10 ml of steatite sphereswith a diameter of 1.8 to 2.2 mm. The preliminary bed used was 14 ml ofsteatite spheres with a diameter of 1.8 to 2.2 mm, which filled theremaining volume of the reactor.

The catalyst was first reduced with 90 l/h of nitrogen and 10 l/h ofhydrogen at 230° C. for one hour. The gas composition selected for theexperiment is typical of the output of the low temperature shift stageafter the reforming of methane, and was 33% by volume of H₂, 28% byvolume of N₂, 25% by volume of H₂O, 13% by volume of CO₂, 0.5% by volumeof CO and 0.5% by volume of CH₄. A loading of 5000 l·h⁻¹·l⁻¹ _(cat) wasselected.

Once all gases had been set and the reactor (after the reduction at 230°C.) had been cooled to 150° C., the experiment was started. Every threehours, the temperature was increased stepwise; the maximum temperaturewas 300° C. The concentration of the gases was determined downstream ofthe reactor by means of GC and IR.

The catalysts were analyzed under the conditions specified.

A selectivity greater than 60% was considered to be satisfactory. Theselectivity falls with rising temperature. Table 1 below reports in eachcase the temperatures at which the selectivity goes below this parameterand the temperature from which CO is depleted to below 10 ppm. The lastcolumn reports the size of the temperature window in which bothsufficient activity (which leads to less than 10 ppm of CO downstream ofthe reactor) and sufficient selectivity (>60%) are achieved.

TABLE 1 List of selected and tested catalysts Temperature [° C.],Temperature from which range [K], with Activity CO <10 ppm Activematerial Selectivity CO <10 and selectivity Catalyst Support <60%ppm >60% Example 1: 3% Ru >260 210 >50  LaCeZr oxide (type I) Example 2:3% Ru >260 210 >50  LaCeZr oxide (type II) Example 3: 3% Ru 265 200 65 LaCeZr oxide (type III) Example 4: 3% Ru and 5% La >260 —* 0 ZSM-5zeolite Example 5: 3% Ru and 1% Fe 245 225 20  ZSM-5 zeolite Example 6:5% Ru 225 —* 0 TiO₂ Example 7a: 5% Ru 245 175 70,  LaCeZr oxide (typeIII) Example 7b: 4% Ru 245 190 55  LaCeZr oxide (type III) Example 7c:3% Ru 265 200 65  LaCeZr oxide (type III) Example 7d: 2% Ru >280220 >60  LaCeZr oxide (type III) Example 8 3% Ru >260 220 40  β-zeoliteExample 9 3% Ru 200 200 0 ZrO₂ Example 10 3% Ru >260 —* 0 Al₂O₃ Example11 3% Ru — —* 0 TiO₂ *Target value of 10 ppm is not attained

FIG. 1 shows the activity and selectivity of selected Ru catalysts inthe temperature screening (performance as a function of differentLaCeZrO_(x) support with the same Ru content). The broad temperaturerange in which the inventive catalysts exhibit full CO conversion whilemaintaining the necessary high selectivity is evident.

FIG. 2 shows the activity and selectivity of selected Ru catalysts inthe temperature screening (performance as a function of the Ru content).The influence of the Ru concentration on the activity range of theindividual catalysts while maintaining a high selectivity is evident.

FIG. 3 shows the activity and selectivity of selected Ru catalysts ondifferent support materials. This demonstrates the superiority of theinventive catalyst based on lanthanum-cerium-zirconium oxide as asupport material over prior art catalysts.

FIG. 4 shows activity and selectivity of selected Ru catalysts in along-term test.

FIG. 5 demonstrates the significantly higher activity and selectivity ofthe inventive catalyst compared to the prior art and existing patentapplications.

1. A process for selective methanization of carbon monoxide in hydrogenand carbon dioxide-containing streams, which comprises carrying out saidselective methanization in the presence of a catalytically activecomposition which comprises ruthenium as the active component and alanthanum-cerium-zirconium oxide as the support material, where thetotal loading of the support material with the active component is 0.1to 20% by weight, based on the total weight of the catalytically activecomposition, and the support material comprises a lanthanum oxidecontent of 0.1 to 15% by weight, a cerium oxide content of 0.1 to 15% byweight and a zirconium oxide content of 30 to 99.8% by weight, based onthe weight of the overall support material.
 2. The process according toclaim 1, wherein the total loading of the support material with theactive component is 0.1 to 10% by weight, based on the total weight ofthe catalytically active composition.
 3. The process according to claim1, wherein the total loading of the support material with the activecomponent is 0.1 to 5% by weight, based on the total weight of thecatalytically active composition.
 4. The process according to claim 1,wherein the total loading of the support material with the activecomponent is 1 to 10% by weight, based on the total weight of thecatalytically active composition.
 5. The process according to claim 1,wherein the total loading of the support material with the activecomponent is 1 to 5% by weight, based on the total weight of thecatalytically active composition.
 6. The process according to claim 1,wherein the total loading of the support material with the activecomponent is 2 to 5% by weight, based on the total weight of thecatalytically active composition.
 7. The process according to claim 1,wherein the catalytically active composition comprises alanthanum-cerium-zirconium oxide support with a lanthanum oxide contentof 10 to 15% by weight and a cerium oxide content of 3 to 7% by weight,based in each case on the weight of the overall support material, and 3to 5% by weight of ruthenium, based on the total weight of thecatalytically active composition.
 8. The process according to claim 1,wherein the methanization is effected in a temperature range from 100 to300° C.
 9. The process according to claim 1, wherein the methanizationis effected in a temperature range from 180 to 260° C.
 10. The processaccording to claim 8, which is carried out directly following a lowtemperature conversion stage.
 11. The process according to claim 10,which is carried out directly following a low temperature conversionstage.